1. Field of the Invention
The present invention is directed to a communication cable which has at least one glass light waveguide that is provided with a closely fitting casing (usually called a buffer).
2. Prior Art
In previously known waveguide cable constructions, the glass fiber is relieved of external forces as far as possible. For example, it can be loosely positioned within a tube as disclosed in British Pat. No. 1,487,464, which corresponds to German Offenlegungsschrift No. 2,528,991. In another type of construction, the glass fibers are loosely received in cable chambers as disclosed in British Pat. No. 1,409,303, which corresponds to German Offenlegungsschrift No. 2,347,408 or it may be packed in a soft cushioning layer, which may consist, for example, of silicon rubber, PU rubber, softly set polyamides or polyolefines. In either type, the glass fiber is thus protected from the influence of external forces. The reason for using these construction principles is due to the sensitivity of the glass fibers to micro-bends, which lead to a mixing of the light modes and to radiation and thus to an increase in the attenuation of the optical signal being carried by the fiber. For example, a periodic bending of a fiber, which bending has a mechanical wave length of 1 mm and an amplitude of only 1 .mu.m, will lead to an additional attenuation of approximately 1 dB/km. In conventional cable construction in which the fibers rest loosely or are softly cushioned, only slight micro-bends occur and the additional attenuation due to the cabling is generally below 2 dB/km.
However, these constructions do have one disadvantage. The surface cracks, which determine the fiber strength, can slowly grow in the case of permanent tensile stresses being applied to the fiber, which tensile stresses are caused by the cable bend or by tensile force acting on the cable. Thus, in the course of time, this increasing of the size of the cracks due to the tensile stresses can lead to breaking of the light waveguide and the life duration is therefore limited. In conventional cable constructions, this life duration limit or expectancy is extended by permitting only very low permanent stresses on the cable, or by heavily reinforcing the cable by providing tension relief elements. The first solution limits possible uses for the cable and the second solution is extremely expensive. In addition to the above, the surface irregularities, which can form an origin for the fiber breakage or give rise to mutual friction of the optical fiber have been rendered harmless by treating the fiber prior to further processing. For example, the glass fibers may be provided with a wash such as described in U.S. Pat. No. 3,108,891 or a protective casing or coating.